docs: def _get_proc_start_time(self, sentry_unit, service, pgrep_full=None):
          """Get start time of a process based on the last modification time
             of the /proc/pid directory.
  
          :sentry_unit:  The sentry unit to check for the service on
          :service:  service name to look for in process table
          :pgrep_full:  [Deprecated] Use full command line search mode with pgrep
          :returns:  epoch time of service process start
          :param commands:  list of bash commands
          :param sentry_units:  list of sentry unit pointers
          :returns:  None if successful; Failure message otherwise
          """
          pid_list = self.get_process_id_list(
              sentry_unit, service, pgrep_full=pgrep_full)
          pid = pid_list[0]
          proc_dir = '/proc/{}'.format(pid)
          self.log.debug('Pid for {} on {}: {}'.format(
              service, sentry_unit.info['unit_name'], pid))
  
          return self._get_dir_mtime(sentry_unit, proc_dir)